This invention relates to nuclear power reactors, and in particular to structure carried by nuclear fuel assemblies for supporting fuel elements.
It has been the practice for some time to support fuel elements for light water power reactors in fuel assemblies having grids which resiliently urge each fuel rod against a stop surface, thereby providing lateral spacing of the elements while permitting differential thermal expansion of each element in the assembly. A particularly efficient grid of this type is disclosed in U.S. Pat. No. 3,423,287, "Nuclear Reactor Fuel Element Support", wherein the grid strips are bent in alternating directions at intervals corresponding to the fuel element pitch, to form an undulating configuration with the bends serving as relatively rigid stop surfaces. The resilient means are provided by spring tabs integrally formed and projecting from the strips and the perimeter plates that surround the fuel assembly. Relative to other designs, the undulating design is easy to manufacture, since the resilient means and stop surfaces are integrally formed on each strip. Furthermore, the grid is relatively rigid against transverse loads, and presents a relatively small cross-sectional area for minimizing the fuel fluid pressure drop across the grid. Typically, this prior art grid is made entirely of Zircaloy, which is relatively transparent to thermal neutrons.
Recent observations of such all-Zircaloy assemblies removed from the reactor core during refueling, have revealed a persistent tendency for the outer surfaces of the grids to wear. This is due to relative motion and rubbing contact between adjacent spacer grids or core shroud structures, and can be further aggravated by the observed tendency of fuel assemblies to bow or deform, out of their nominally square envelope. It is believed that bowing results from continual asymmetrical lateral flow forces on the assemblies, which in the presence of high radiation, permanently deform the Zircaloy structure. A second observation of the behavior of the prior art grid is its tendency in crush tests, to deform in a predictable fashion to one side or the other.